And it looks like there's two different spellings, Jedadiah and Jedidiah. So the question: Should I spell it the way I say it (Jedadiah, with an 'uh' sound) or spell it so that Jedi is a possible nickname (Jedidiah)?

I do like Jedidiah (it only looks right to me spelled this way) but sadly I can only imagine someone very strange (like a dangerous religious radical) wearing it. I mentioned it to my husband and suggested that we could use nn Jedi. I'm not a Star Wars fan, just nerdy. It was an instant veto. IMO, Jeremiah is a much friendlier alternative.